Simulations for ultra high energy cosmic
Ultra-high energy cosmic
rays (E >10^19 eV) are recorded since more than 40 years. They
pose very fundamental physics questions which are still
unanswered today: Where do they come from? What are they? How
are they accelerated to these ultra-high energies?
To find answers is difficult because of their extremely low flux
(only about 1 particle per km^2 and year) and because charged
particles are deflected in cosmic magnetic fields and do,
therefore, not point back to their sources.
At those fluxes cosmic rays can only be measured indirectly,
i.e. by instrumenting huge natural volumes, such as the
atmosphere, sea water or ice, in order to detect the secondary
particle showers the cosmic rays induce when interacting. This
approach, however, requires to reconstruct the primary
particle's energy and mass from the shape and the particle
content of the shower.
Usually the reconstruction compares the measured showers with
those simulated with an elaborate computer model of the shower
development for different energies and primaries. The most
crucial part of these simulations are the hadronic and nuclear
interactions of the primary and secondary particles with the
nuclei of the atmosphere. Since hadronic interactions (unlike
electromagnetic ones) cannot be predicted from fundamental
theories, and since energies in cosmic ray shower are orders of
magnitude higher than what is available at man-made accelerators,
the interaction models are phenomenological and are the main
source of systematic errors.
This lecture will describe the main features of shower
simulation programs, discuss their capabilities and limitations,
and explain how they are used to interpret cosmic ray data.